Catalyst for solid polymer fuel cells and method for manufacturing the same
11239473 · 2022-02-01
Assignee
Inventors
Cpc classification
Y02E60/50
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
International classification
H01M4/86
ELECTRICITY
Abstract
The present invention relates to a catalyst for solid polymer fuel cells in which catalyst particles including platinum or platinum alloy are supported on a carbon powder carrier. The catalyst of the present invention is a catalyst for solid polymer fuel cells in which the bond energy (Ec) at a gravity center position is 2.90 eV or more and 3.85 eV or less as calculated from a spectrum area of a Pt5d orbit-derived spectrum which is obtained by measuring a valence band spectrum in a range of 0 eV or more and 20 eV or less in the result of subjecting the catalyst particles to X-ray photoelectron spectroscopic analysis.
Claims
1. A catalyst for solid polymer fuel cells in which catalyst particles consisting of platinum or a single phase platinum alloy are supported on a carbon powder carrier, wherein a bond energy (Ec) at a gravity center position is 2.90 eV or more and 3.85 eV or less as calculated from a spectrum area of a Pt5d orbit-derived spectrum which is obtained by subjecting the catalyst particles to X-ray photoelectron spectroscopic analysis to measure a valence band spectrum in a range of 0 eV or more and 20 eV or less wherein the X-ray photoelectron spectroscopic analysis uses a monochromatized Al-KαX-ray source.
2. The catalyst for solid polymer fuel cells according to claim 1, wherein the bond energy (Ec) at the gravity center position is 2.95 eV or more and 3.78 eV or less.
3. The catalyst for solid polymer fuel cells according to claim 1, wherein the bond energy (Ec) at the gravity center position is 2.95 eV or more and 3.65 eV or less.
4. The catalyst for solid polymer fuel cells according to claim 1, wherein the bond energy (Ec) at the gravity center position is 2.95 eV or more and 3.53 eV or less.
5. The catalyst for solid polymer fuel cells according to claim 1, wherein a ratio of zerovalent platinum to platinum present on surfaces of the catalyst particles is 95% or more and 100% or less.
6. The catalyst for solid polymer fuel cells according to claim 1, wherein a ratio of zerovalent platinum to platinum present on surfaces of the catalyst particles is 100%.
7. The catalyst for solid polymer fuel cells according to claim 1, wherein a ratio of zerovalent platinum to platinum present on surfaces of the catalyst particles is 75% or more and 100% or less.
8. The catalyst for solid polymer fuel cells according to claim 7, wherein the catalyst particles have a particle size of 2 nm or more and 500 nm or less.
9. The catalyst for solid polymer fuel cells according to claim 7, wherein a carrying rate of the catalyst particles is 20% or more and 70% or less.
10. The catalyst for solid polymer fuel cells according to claim 1, wherein the catalyst particles have a particle size of 2 nm or more and 500 nm or less.
11. The catalyst for solid polymer fuel cells according to claim 10, wherein a carrying rate of the catalyst particles is 20% or more and 70% or less.
12. The catalyst for solid polymer fuel cells according to claim 1, wherein a carrying rate of the catalyst particles is 20% or more and 70% or less.
13. The catalyst for solid polymer fuel cells according to claim 1, in which the catalyst particles consist of platinum.
14. The catalyst for solid polymer fuel cells according to claim 1, in which the catalyst particles consist of the single phase platinum alloy.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1)
(2)
(3)
DESCRIPTION OF EMBODIMENTS
First Embodiment
(4) Hereinafter, a preferred embodiment of the present invention will be described. In this embodiment, a platinum catalyst was manufactured in which only platinum was supported, and properties of the platinum catalyst were evaluated.
Example 1
(5) A platinum catalyst was manufactured in the following manner. First, in a colloid mill, a dinitrodiammine platinum nitric acid solution (platinum content: 30.8 g) was diluted with pure water to prepare 4685 mL of an aqueous solution.
(6) 30.8 g of carbon fine powder (specific surface area: 800 m.sup.2/g, trade name: OSAB) as a carrier was added to the dinitrodiammine platinum nitric acid aqueous solution while being ground. Grinding treatment was performed for 1 hour, a denatured alcohol (95% of ethanol+5% of methanol) as a reducing agent was then added in an amount of 318 mL (6.4% by volume, 34.5 mol based on 1 mol of platinum), and mixed. The mixed solution was refluxed and reacted at about 95° C. for 6 hours to reduce the platinum. Thereafter, filtration, drying and washing were performed. By the above steps, a platinum catalyst was obtained in which platinum particles were supported. The carrying rate in the catalyst was 48%, and the average particle size of the catalyst particles was 2.5 nm.
Comparative Example
(7) A platinum catalyst similar to that in Example 1 was manufactured. A carbon fine powder carrier was introduced into a dinitrodiammine platinum nitric acid solution identical to that in Example 1, and rather than grinding treatment, stirring was performed to prepare a slurry. The slurry was brought into a neutral condition, so that platinum was precipitated to manufacture a platinum catalyst. The carrying rate in the catalyst was 45%, and the average particle size of the catalyst particles was less than 2.0 nm.
(8) The platinum catalysts manufactured in Example 1 and Comparative Example were subjected to XPS analysis, and the d band center values and the states of platinum on the surface (ratios of zerovalent platinum) were evaluated. For the XPS analysis, Quantera SXM manufactured by ULVAC-PHI, Inc. was used as an analyzer. For analysis, a platinum catalyst was fixed on a vacuum double-sided carbon tape as preparation of a sample. Here, a sufficient amount of the platinum catalyst was placed so as not to expose a backing tape portion, and the platinum catalyst was pressed from above a powder paper to form a flat surface. Thereafter, an excess sample was removed by a blower. As pretreatment of the sample, sputter etching was performed by use of an XPS-attached ion gun for evaluating a state in which surface contaminants of the platinum catalyst were removed. As a sputter condition, Ar ions were applied to the catalyst at an accelerating voltage of 1 kV (1 min).
(9) As XPS analysis conditions, a monochromatized Al-Kα ray was used as an applied X-ray, the voltage was 15 kV, the power was 25 W, and the X-ray beam diameter was 200 μmφ. Generated photoelectric energy was detected to acquire a wide-area photoelectron spectrum (wide spectrum) and a narrow-area photoelectron spectrum (narrow spectrum).
(10) The data of the obtained spectrum was analyzed by use of software (MultiPak V8.2C) manufactured by ULVAC-PHI, Inc. The analysis of the d band center was performed by use of the Pt5d spectrum in XPS. In the analysis, first, the measured Pt5d spectrum was corrected by software so that a C—C-derived peak appeared at 284.8 eV in the C1s spectrum. A background was subtracted by the Iterated Shirley method, and the bond energy value at which the spectrum area after correction was divided in half was defined as a d band center value (Ec).
(11) Further, the result of measuring the Pt4f spectrum in XPS was used to determine the ratio of zerovalent platinum to platinum on the surfaces of the catalyst particles. In this analysis, “Pt” was associated with three chemical states (zerovalent Pt (0), divalent Pt (II) and tetravalent Pt (IV)). The main peak positions for the states were set at 71.7 eV for zerovalent Pt (0), 72.7 eV for divalent Pt (II) and 74.4 eV for tetravalent Pt (IV), and separation of peaks in the Pt4f spectrum measured by the software was performed. After the separation of peaks was performed, the ratio of each Pt was calculated from the area ratio of the peak for each state.
(12) From the XPS spectrum measured for the platinum catalysts of Example 1 and Comparative Example, the d band center values of the platinum catalysts were calculated, and the results showed that the d band center values in Example 1 and Comparative Example were 3.78 eV and 3.99 eV, respectively. Further, the ratios of zerovalent platinum (Pt(0)) in Example 1 and Comparative Example were 79% and 47%, respectively. In the platinum catalyst of this embodiment, the d band center position shifts to the negative side as compared to Comparative Example. Further, it was found that a ratio of zerovalent platinum was higher.
(13) Next, catalytic activity (initial activity) was evaluated for the platinum catalysts of Example 1 and Comparative Example. In this evaluation test, the area specific activity was calculated from the measured values of the electrochemical surface area and the activity controlling current of each catalyst was calculated, and evaluated as a relative value against the area specific activity in Comparative Example. The electrochemical surface area was measured in the following manner. The amount of electricity in adsorption and desorption of flowing protons at a potential cycle between 0.05 V and 1.2 V (sweeping rate: 100 mV/s) in an electrolytic solution (0.1 M perchloric acid) in which a rotating disc electrode coated with 8 μg of a catalyst was saturated with nitrogen. From the measured amount of electricity in adsorption and desorption of protons, the electrochemical surface area was calculated by use of a constant (0.21 mC/cm.sup.2).
(14) The method for measuring the activity controlling current includes rotating the rotating disc electrode to examine oxygen reduction activity. Specifically, in an electrolytic solution (0.1 M perchloric acid) saturated with oxygen, the electrode was steadily rotated (400 rpm, 900 rpm, 1600 rpm, 2500 rpm, 3600 rpm), and the oxygen reduction current was measured at a sweeping rate of 20 mV/s with the voltage changed from 0.5 V to 1.0 V. After the measurement, the current value at 0.9 V at each rotation speed was approximated by the Koutecky-Levich equation to obtain an activity controlling current. The area specific activity was calculated by use of the electrochemical surface area obtained as described above. In this embodiment, the area specific activity of the catalyst of Example 1 was determined as a relative value against the area specific activity in Comparative Example which was defined as “1”. The result of the evaluation is shown in Table 1 together with the above-described physical property values.
(15) TABLE-US-00001 TABLE 1 d band center Pt ratio (Ec) Pt (0) Pt (II) + Pt (IV) Initial activity Example 1 3.78 eV 79% 21% 1.75 Comparative 3.99 eV 47% 53% 1 Example
(16) Table 1 reveals that the platinum catalyst in which the d band center value in Example 1 had higher initial activity.
Second Embodiment
(17) Here, a plurality of platinum catalysts and platinum alloy catalysts were manufactured, and in the same manner as in the first embodiment, the d band center value (Ec) and the ratio of zerovalent platinum (Pt(0)) were evaluated, and an activity test was conducted.
Example 2
(18) The platinum catalyst manufactured in Example 1 was further subjected to heat treatment to manufacture a catalyst. The heat treatment was performed by heating the platinum catalyst in a hydrogen reducing atmosphere at 900° C. for 1 hour. The carrying rate in the catalyst was 51%, and the average particle size of the catalyst particles was 4.7 nm.
Example 3
(19) In this example, two stages of platinum supporting steps were carried out to manufacture a platinum catalyst. In the first-stage platinum supporting step, a platinic chloride solution (platinum content: 30.8 g) was diluted with pure water to prepare 4685 mL of an aqueous solution. 13.2 g of a carbon fine powder carrier identical to that in Example 1 was added while being ground. Grinding treatment was performed for 1 hour, and the pH was then adjusted with sodium hydroxide, and a reduction reaction was carried out at about 70° C. for 2 hours. Methanol as a reducing agent was added to and mixed with the reaction solution. The mixed solution was subjected to a reduction reaction at about 70° C. for 2 hours to reduce the platinum. Thereafter, filtration, drying and washing were performed.
(20) Subsequently, in the second-stage platinum supporting step, a carrier having platinum supported in the first stage was added to a dinitrodiammine platinum nitric acid aqueous solution (4685 mL, platinum content: 30.8 g) to that in Example 1. By adding methanol as a reducing agent, a reduction reaction was carried out at 70° C. for 1 hour to reduce the platinum. A platinum catalyst was manufactured through the above first-stage and second-stage platinum supporting steps. The carrying rate in the catalyst was 48%, and the average particle size of the catalyst particles was 2.1 nm.
Example 4
(21) In this example, a catalyst having platinum-cobalt alloy particles supported as catalyst particles was manufactured. 10 g of the platinum catalyst manufactured in Example 1 was immersed in 60 g of a cobalt chloride aqueous solution (cobalt content: 0.4 g) having a cobalt concentration of 0.66 wt % as a cobalt solution. The solution was stirred for 1 hour, dried at 60° C., and then heated in a hydrogen reducing atmosphere at 900° C. for 1 hour. The carrying rate in the catalyst was 52%, and the average particle size of the catalyst particles was 3.5 nm.
Example 5
(22) In this example, a catalyst including catalyst particles having a so-called core/shell structure in which platinum was precipitated on the surfaces of palladium particles. First, 35 g of carbon powder (trade name: Ketjen Black EC, specific surface area: 800 g/m.sup.3) as a carrier was immersed in a palladium chloride solution (Pd content: 15 g (0.028 mol)) in a colloid mill, and then made neutral with sodium carbonate. This carbon powder was reduced with sodium formate to prepare carbon powder in which palladium particles as core particles were supported. The carrying rate in the catalyst was 50%, and the average particle size of the catalyst particles was 4.5 nm.
(23) Next, platinum was supported by a Cu-UPD method. In this method, first the surfaces of the palladium particles were covered with a copper layer by use of an electrolyzer. The electrolyzer used in this embodiment is shown in
(24) In the electrolytic treatment of the palladium particles, first, 6 L of a sulfuric acid solution (0.05 M) was added into an electrolytic bath, and 50 g (0.32 mol) of copper sulfate was dissolved in this solution to perform pretreatment for reducing the amount of dissolved oxygen. In this pretreatment, first, nitrogen was injected into a glove box to turn the oxygen concentration to about 0 ppm, and with the electrolyzer placed in the glove box, injection of nitrogen into the glove box and bubbling of the electrolytic solution with nitrogen were performed for 12 hours. The amount of dissolved oxygen was confirmed to be 1 ppm or less before the electrolytic treatment. 10 g of carbon powder with palladium particles supported in the manner described above was put to the bottom portion of the electrolytic bath, and potential control was performed by the potential controller to electrolytically precipitate copper. Electrolysis conditions here are as follows. During the electrolytic treatment, injection of nitrogen into the glove box and bubbling of the electrolytic solution with nitrogen were continued.
(25) [Electrolysis Conditions] Potential: fixation of potential at 0.39 V (vs. RHE) Potential fixation time: 3 hours
(26) In the electrolytic bath, 3.4 g (0.0083 mol) of potassium platinichloride was dissolved as a platinum compound solution after the electrolytic treatment. Further, at the same time, 48 g of citric acid was added. This causes a displacement reaction between copper on the surfaces of palladium core particles and platinum. The reaction time of the displacement reaction step was 1 hour. After formation of a platinum shell layer, the carbon powder was recovered by filtration, washed with pure water, and dried at 60° C. to obtain a catalyst.
(27) The catalyst of each of Examples 2 to 5 manufactured in this embodiment was subjected to XPS analysis to determine the d band center value (Ec) and the ratio of zerovalent platinum (Pt(0)). Further, an activation test was conducted. The evaluation and test conditions were the same as in the first embodiment. The results thereof are shown in Table 2.
(28) TABLE-US-00002 TABLE 2 d band center Pt ratio (Ec) Pt (0) Pt (II) + PT (IV) Initial activity Example 1 3.78 eV 79% 21% 1.75 Example 2 3.53 eV 100% 0% 2.24 Example 3 3.65 eV 95% 5% 2.51 Example 4 3.51 eV 100% 0% 4.45 Example 5 2.95 eV 100% 0% 3.94 Comparative 3.99 eV 47% 53% 1 Example
(29) The catalysts of examples were shown to have a d band center value (Ec) of 3.85 eV or less and exhibit favorable initial activity. Comparison between Example 1 and Example 2 shows that the ratio of zerovalent platinum (Pt(0)) increases. This may be because oxide-derived divalent or tetravalent platinum was reduced by high-temperature treatment after platinum was supported. It was confirmed that an increase in the amount of zerovalent platinum enhanced activity.
INDUSTRIAL APPLICABILITY
(30) The catalyst for solid polymer fuel cells according to the present invention is excellent in an initial activity because the electron state of platinum in catalyst particles is optimized. Solid polymer fuel cells are expected as future power generation systems for automobile power sources and domestic power sources, and commercialization of fuel cell mounting vehicles has been started. The present invention is an invention which contributes to promotion of practical realization of the fuel cells.